The present invention relates to a honeycomb core material for a sandwich structure and a method for its production.
Heretofore, a sandwich structure having a honeycomb core material sandwiched by rigid surface materials made of e.g. a steel plate, has been widely used as an interior or exterior panel for a building material such as a door or a partition, or as a material for a structure such as an aircraft or a vehicle, since it is excellent in such characteristics as light weight, rigidity, sound absorption and thermal insulation. In order to improve the sound absorption or thermal insulation among such various characteristics of the structure, a honeycomb core material having a resin foam filled in cells of the honeycomb body, has also been known.
However, with a conventional honeycomb core material having a resin foam filled, the resin foam filled in cells was hard and brittle, and if bending occurred by e.g. transportation, the foam was likely to slip off from the cells, and consequently, the transportability or handling efficiency was poor. Further, in a state where the resin foam was not filled over the entire cells of the honeycomb core material, the thermal insulation of the honeycomb core material was non-uniform or poor. Therefore, JP-A-1-301329 proposes to prevent slipping off of the foam by providing a notch at one end of the cell wall of the honeycomb core material. However, in this case, the partition wall was not continuous, and there was a problem that the mechanical strength, particularly the shear strength, was poor.
On the other hand, PCT publication WO97/11925 discloses a foam of a phosphorus-containing acid material as one which is excellent in non-combustibility and fire proof property and whereby the foam can be molded under ordinary temperature and ordinary pressure conditions. However, such a foam was flexible and had elasticity, and by itself, it was sometimes inadequate as a structure whereby rigidity was required.
An object of the present invention is to provide a honeycomb core material whereby a resin foam is flexible and has elasticity while maintaining the thermal insulation and noise insulation of a honeycomb core material having a conventional resin foam filled, and which is suitable for a sandwich structure having improved transportability and handling efficiency and useful also for a curved sandwich structure, and a method for its production.
The present invention has been made to accomplish the above object. Namely, the present invention resides in a honeycomb core material for a sandwich structure, which comprises a honeycomb body and a filler filled in at least a part of cells thereof, wherein the honeycomb body has a cell size of from 3 to 100 mm and a porosity of from 92 to 99.5%, as stipulated in JIS-A6931, and the filler is a composite foam comprising an inorganic foam obtained from a phosphorus-containing acid material (a), a curing agent (b) and, if necessary, a blowing agent (c), and an organic foam obtained from a urethane prepolymer having NCO groups (d), and water (e).
Further, the present invention resides in a method for producing a honeycomb core material for a sandwich structure, which comprises filling a mixture comprising a phosphorus-containing acid material (a), a curing agent (b), a urethane prepolymer having NCO groups (d), water (e) and, if necessary, a blowing agent (c), in cells of a honeycomb body having a cell size of from 3 to 100 mm and a porosity of from 92 to 99.5%, as stipulated in JIS-A6931, followed by foaming and curing.
Such a honeycomb core material according to the present invention is excellent in flame retardation or non-flammability which can not be attained by a conventional organic foam and is flexible and has elasticity, which can not be obtained by a conventional inorganic foam, as the filler in its cells is composed of the above-mentioned inorganic and organic composite foam. In addition, according to the discovery by the present inventors, it has been found that when components to form such a composite foam, are foamed and cured in cells of the honeycomb body, the foaming and expansion force of the foam bring about an extremely high adhesiveness between the honeycomb and the filler in combination with the inherently high adhesiveness of a urethane polymer.
The honeycomb core material of the present invention is thereby free from slipping off of the filler from the honeycomb body and easy in transportation or handling, and also has flexibility and modification in shape after the production is possible, whereby surface materials such as flat plates can easily be attached to the honeycomb core material, and it becomes possible to apply it to a structure having a curved shape.